The present invention relates generally to the sextant arts, and more particularly pertains to a new and improved sextant which utilizes a digital encoder to obtain a digital output signal which is an exact indication of the sextant reading.
A sextant is an optical instrument used by a navigator to determine position on the earth. The instrument basically measures the altitude of a celestial body, which is the angular position of that body above the horizon. Several altitude measurements relative to several celestial bodies are then utilized to solve multiple algebraic equations. These equations are derived from spherical geometry, and a navigational triangle is solved for the measurement of each celestial body. Even today, this solution supplies the navigator with the most accurate measurement of his position (latitude and longitude) currently obtainable. Today, most of the mathematics have been precomputed, and the navigator need only consult his Sight Reduction Tables, do some simple addition and subtraction, and finally plot the results on a chart to determine his position on earth.
The major problem with present sextants is accuracy. The accuracy of the sextant sightings determine the accuracy of the finally computed position on earth. The accuracy problem is multifold; the first problem is the inherent accuracy of the instrument; the second problem is the accuracy of the navigator's sighting of the instrument, which is often encumbered by adverse conditions such as movement, vibration, oscillation, etc. In the art today, the basic instrument accuracy of a standard marine sextant is about 5 seconds arc, but the navigator can only interpolate a reading to 12 seconds of arc and can only sight and align the instrument to about 18 seconds arc. Because of difficulty in sighting the sextant, mechanical instruments, which average a number of sights over a time period, have been used to overcome this problem. Such a mechanically averaging sextant is illustrated in U.S. Pat. No. 2,140,579, for Averaging Device, by Gray. With that sextant, the readout is a mathematical average of a number of sights taken over a given time period. One disadvantage of such a sextant is that it inherently has low accuracy (approximately 2 minutes arc) which limits its use to aircraft applications. Other limiting features of such an instrument are that the output is in the form of a mechanical reading, additional arithmetic calculations must be made, and finally the operator must still consult his Sight Reduction Tables and chart the results.